Many processes are known and described in the literature for preparing polyisocyanates by phosgenation of the corresponding amines. Depending on the type of amines, the reaction can be carried out in the gas or liquid phase and batchwise or continuously (W. Siefken, Liebigs Ann. 562, 75-106 (1949)).
The procedure for continuous syntheses of organic isocyanates on an industrial scale has already been described a number of times, see, for example, Ullmanns Encyklopädie der technischen Chemie, 4th edition (1977), volume 13, pp. 351 to 353. Both aromatic isocyanates such as methylenedi(phenyl isocyanate) (hereinafter MMDI—“monomeric MDI”), polymethylene-polyphenylene polyisocyanate (a mixture of MMDI and its higher homologs, hereinafter PMDI, “polymeric MDI”) to tolylene diisocyanate (hereinafter TDI) and also aliphatic isocyanates such as hexamethylene diisocyanate (hereinafter HDI) or isophorone diisocyanate (hereinafter IPDI) are used worldwide.
The industrial processes for the production of aromatic isocyanates such as MMDI, PMDI and TDI and of aliphatic isocyanates such as HDI and IPDI are virtually exclusively operated in the continuous mode. DE-A-844 896 may be mentioned as an example of such a process in various continuously operated vessels.
The phosgenation of primary amines (RNH2) is usually carried out in stages, with the carbamoyl chloride (RNHCOCl) firstly being formed from the starting materials at low temperature and this subsequently being converted at elevated temperature into the corresponding isocyanate (RNCO), and with hydrogen chloride being eliminated in both steps. During the first stage, known as the “cold phosgenation”, the amine hydrochloride (“RNH2.HCl”=RNH3Cl) corresponding to the amine used occurs as significant by-product, and this reacts in the “hot phosgenation” in the presence of phosgene to form the corresponding isocyanate. Temperatures below 60° C. are usually employed in the cold phosgenation, while temperatures in the range from 100° C. to 200° C. are attained in the case of the hot phosgenation. Two-stage processes are described, for example, in the documents DE-A-20 58 032, DE-A-21 53 268 and DE-A-1 233 854.
At all temperatures and pressures employed industrially, the reaction between amine and phosgene occurs very quickly in the liquid phase. In order to avoid secondary reactions, the mixing of the reactants should be carried out very effectively. The phosgenation of primary amines in a mixer-reactor as first stage has therefore been disclosed in many publications.
Mixers can be divided into various classes. Apart from dynamic mixers (e.g. stirrers, turbines or rotor-stator systems) and static mixers such as Kenics, Schaschlik or SMV mixers, nozzle mixers are also known (Ind. Eng. Chem. Res. 26, 1987, 1184-1193). For example, pin mixers (EP-A-2 077 150) and Lefos nozzles (EP-A-0 322 647) are particularly suitable for preparing aromatic isocyanates.
A number of apparatuses have been developed for the phosgenation of amines, with these optionally also being able to be used as phase separation vessels. The phosgenation of amines to form the corresponding isocyanates can therefore take place in a stirred vessel (e.g. DE-A 14 68 445), in a cascade of stirred vessels (e.g. DE-C 844 896) or in tube reactors, with the latter being able to be either packed (e.g. WO-A-99/54289) and unpacked (e.g. Ullmanns Encyklopädie der technischen Chemie, 4th edition (1977), volume 13, pp. 351-353). In the case of a reduced reactor volume, circulation reactors with recirculation can also be used to ensure a sufficient residence time for completing the reaction.
The first publication DE-A-1 593 412 fundamentally describes a continuous production process for organic isocyanates, which comprises a “reaction circle” (in the drawing the tube 3 present in the form of a ring conduit) which is followed by the work-up by distillation in a second reaction stage in a column in which the carbamoyl chloride present is converted into the corresponding isocyanate. Apart from MMDI, the preparation of TDI and aliphatic isocyanates was also described. In the reaction circle, the reaction of amine to form carbamoyl chloride is carried out at a pressure of from 10 to 50 atm gauge (about 11 to 51 bar absolute) and a temperature of from 40 to 120° C. The reaction circle is operated with an amine stream being introduced at one point on the ring conduit and a mixture of fresh phosgene and phosgene recovered in the column being introduced at another position on the reaction circle located downstream of the place of introduction of the amine. The starting materials introduced in this way and carbamoyl chloride formed therefrom are circulated in the ring conduit. Part of the circulated reaction mixture is discharged each time unit at a third position on the reaction circle located downstream of the place of introduction of phosgene via a separator integrated into the reaction circle and is fed to the column. The use of a mixing device different from a single tube, in which amine, phosgene and carbamoyl chloride are mixed at the same time, is not disclosed. Phosgene is preferably used in a very large excess, for example from 100 to 500%, probably in order to suppress polymerization reactions. The column mentioned is considered to be an apparatus for dissociating carbamoyl chloride into isocyanate and hydrogen chloride and is operated at a pressure of at least 10 atm gauge (about 11 bar absolute). Below the top of the column, phosgene is recovered in a side offtake stream and is recirculated via a stop vessel into the reaction circle. At the top of the column, a hydrogen chloride condensate (about 10 kg/h) which still contains about 6% of phosgene is obtained via heat exchangers at 82° C. (example 3). The MMDI solution obtained at the bottom at 142° C. is then fed to the solvent rectification column; this stream contains not only MMDI (14.9 kg/h) but also still considerable amounts of phosgene (30 kg/h).
A circulation process with subsequent work-up for the preparation of isocyanates by phosgenation is disclosed in EP 0 716 079 B1. The design of the process advantageously allows the omission of a circulation pump. Excesses of phosgene in the range from 110 to 300% are claimed. The starting materials are fed at separate places into the bubble column, with the phosgene being present in gaseous form and the mixture consisting of MDA and monochlorobenzene being present in liquid form. The reaction solution is continuously circulated by the evolution of hydrogen chloride. The process is operated in a temperature range from 60 to 100° C. and at a pressure of from 0.5 to 5 bar.
A circulation reactor which is operated at pressures of up to 14 kg/cm2 is described by DE-B-1 037 444. Here, amine, phosgene and the inert solvent o-dichlorobenzene are fed into the mixing circle at three different places, with a pump effecting the circulation mode. After the abovementioned streams have been combined, the reaction mixture goes into a heater and temperatures above 110° C. are attained. The mixture is depressurized via a throttle valve and then goes into a collection vessel which is operated under atmospheric pressure. The gaseous materials are taken off at the top of the collection vessel and obtained as a mixture consisting of hydrogen chloride and phosgene via a condenser. At the bottom of said vessel, part of the isocyanate solution is recirculated and the other part is subjected to a further separation operation. Excesses of phosgene of at least 96% are necessary in order to obtain a yield of diphenylmethane 4,4′-diisocyanate of 90.5% in the reactor described at a low gauge pressure of 0.07 kg/cm2.
A two-stage production process for isocyanates is described in DE 32 12 510 C3, with a mixture consisting of isocyanates and the corresponding carbamoyl chloride being recirculated in the presence of phosgene and an inert solvent. The first reaction stage is carried out in a tank-like vessel or in a tubular circulation conduit at temperatures of from 60 to 100° C. and an absolute pressure of from 4 to 8 bar. To complete the conversion of carbamoyl chloride formed as an intermediate, the reaction mixture is fed at the same pressure but at an increased temperature of from 120 to 160° C. into a second stage in order to obtain an isocyanate concentration of from 10 to 25%. Due to the reaction conditions selected, the plant described can advantageously be lined with stainless steel instead of more costly materials. The reaction mixture is present as a slurry (=suspension) in the process described. The excess of phosgene described is at least 100%, and hydrogen chloride is discharged at a maximum of 10.8 bar.
To combine the starting materials with recirculated reaction mixture, DE 26 24 285 C2 describes the use of a motive jet nozzle which advantageously allows intensive mixing in a short time. As a result of the pressure range from 1 to 10 bar employed, not only crude MDI but also a tolylene diisocyanate isomer mixture, naphthalene 1,5-diisocyanate and phenylene 1,4-diisocyanate could be obtained in high yields at residence times of from 10 to 180 minutes. No pressure increase to increase the yield was found, and the excess of phosgene relative to MDA as amine component was greater than 100%.
DE-A- 2 252 068 describes a process for the phosgenation of amines to form isocyanates, which is operated without solvent and with recirculation of the isocyanates produced at a pressure of up to 100 atm and a temperature of up to 240° C. in an apparatus. Here, liquefied amine is firstly reacted adiabatically at 100 atm and 150° C. with a mixture consisting of phosgene and recirculated isocyanate in a tube reactor, with a temperature of 240° C. being attained. The reaction mixture is subsequently depressurized isentropically to 20 atm and fresh phosgene/isocyanate mixture is introduced. Apart from isocyanates, hydrogen chloride is obtained at a pressure of 3 atm by work-up of the gaseous components by distillation and phosgene is recirculated in liquid form to the process.
A two-stage process for the production of isocyanate is disclosed by DE-A-2 058 032; here, the temperature is gradually increased during the hot phosgenation. An excess of phosgene of, for example, 8% can be employed here in order to obtain isocyanates in yields in the range from 90 to 95%. This is demonstrated specifically only for the phosgenation of the monofunctional amine aniline. These yields are too low for today's requirements; in addition, it is questionable whether the knowledge disclosed in this document can readily be applied to the phosgenation of polyamines in which there is a risk of polymerization reactions. Particular mention may here be made of the formation of polyureas by polyaddition of polyamines with polyisocyanates. The plant described comprises, as significant part, a mechanically mixed, horizontal tube in which the temperature is gradually increased from 30 to 150° C. and which connects the cold phosgenation part to the degassing tube.
A circulation apparatus for preparing isocyanates, which consist essentially of a circulation conduit, a polyamine/carbamoyl chloride contact unit and a mixing unit for applying shear, is described in EP-A-1 867 632. A distance of 1000 mm and less between contact unit and mixing is claimed. The advantage of the apparatus described is that the formation of ureas as secondary components is reduced by the more effective mixing of the two reactants. Excesses of phosgene of from 0% to 5900% (2≤n(COCl2)/n(polyamine)≤60) are indicated. The subject matter of the invention makes it possible, according to statements in the document, to suppress the formation of urea-like secondary components, which has the effect of increasing the yield of polyisocyanate. However, examples which could demonstrate the advantage claimed are not described in the application. In particular, there is no evidence that industrially acceptable yields are achieved even at low or no excesses of phosgene. A distance of 1000 mm or less between contact unit and mixing unit is claimed; the feed streams are introduced at a linear velocity of from 0.5 to 10 m/s into the reaction solution (0.3 to 5 m/s) via tubes. Shape and construction are indicated as drawing in the application. Furthermore, it is said in the description of the application that the formation of carbamoyl chlorides and polyisocyanates is minimized by reaction of HCl with polyamine to form polyamine hydrochloride. Due to generation of a laminar flow profile, no reaction takes place in the circulation conduit.
A further process for preparing isocyanates is disclosed in EP 0 150 435 B1. Here, hydrogen chloride is separated off before the circulation of the reaction mixture present in the circuit in order to obtain concentrations below 0.5% by weight before the addition of amine. Intermediate salt formation and by-product formation is advantageously suppressed in this process by the removal of the hydrogen chloride and, as a result, the isocyanate concentration in the reactor is increased. The molar ratio of phosgene to amine groups is from 12:1 to 200:1. The hydrogen chloride gas which has been separated off is obtained under a high pressure. The reactants are mixed by means of a motive jet nozzle with the recycle stream, which is mainly isocyanate dissolved in monochlorobenzene, with a temperature of 130° C. and a pressure of 14.5 bar prevailing in the mixing circle. A column is used to separate hydrogen chloride and phosgene, with the phosgene obtained at the bottom being recirculated to the process.
It is common to all the above-described processes that in practice they require very large excesses of phosgene in order to achieve very good yields of polyisocyanates. Particularly in the case of the preparation of MMDI and PMDI, a high excess of phosgene is indispensable in the prior art in order to achieve acceptable yields (which in industrial production should be >99% for economic reasons). However, high excesses of phosgene are not very desirable both for economic reasons and for safety reasons. There was therefore a need for a process for the production of polyisocyanates, in which the excess of phosgene can be kept low without this resulting in other disadvantages (such as reduced yield or increased polymerization tendency with the associated risk of the formation of deposits).